A turbocharged engine may include a waste gate for controlling compressor speed. If compressor speed exceeds a desired speed the waste gate may be opened to reduce compressor speed via reducing the amount of exhaust energy used to rotate the compressor. However, the compressor has inertia which tends to keep the compressor spinning at its present speed. Further, at least some exhaust gas energy may continue to apply force to the turbine that rotates the compressor. Consequently, opening the waste gate may not provide a timely reduction in pressure at an intake manifold throttle.
One way to quickly decrease pressure at the intake manifold throttle is to install a compressor recirculation valve. The compressor recirculation valve may be placed in parallel with the compressor so that output from the compressor may be rerouted back to the compressor input, thereby reducing pressure at the compressor outlet and the engine intake manifold throttle. Nevertheless, the compressor recirculation valve may be exposed to fuel vapors and matter inducted to the engine so that flow through the compressor recirculation valve becomes restricted. If flow through the compressor recirculation valve becomes restricted, it may be more difficult to control pressure upstream of an intake manifold throttle inlet. As a result, engine air-fuel ratio control may be more difficult.
The inventors herein have recognized the above-mentioned issues and have developed a diagnostic method, comprising: at least partially opening a waste gate and adjusting a compressor recirculation valve to a closed position in response to a diagnostic request; at least partially closing the waste gate after at least partially opening the waste gate; adjusting a compressor recirculation valve airflow offset in response to a position the compressor recirculation valve is adjusted to maintain a central throttle inlet pressure responsive to the at least partially closing the waste gate; and operating the compressor recirculation valve in response to the airflow offset.
By adjusting a compressor recirculation valve airflow offset in response a position of the compressor recirculation valve after a waste gate is at least partially closed, it may be possible to provide the technical result of improving engine air intake pressure control even when deposits are formed within a compressor recirculation valve. For example, the compressor recirculation valve may be operated in a closed loop mode in response to a request to adapt operation of the compressor recirculation valve. Closing the waste gate increases compressor output so that flow through the compressor recirculation valve needs to be increased to maintain a constant pressure at the engine intake manifold or central throttle inlet. In closed loop control, the compressor recirculation valve opens to maintain the constant pressure at the engine intake manifold throttle inlet. The amount that the compressor recirculation valve has to open to return engine intake manifold throttle inlet pressure to its initial value is monitored. A non-degraded compressor recirculation valve will open a small amount while a degraded compressor recirculation valve will need to open a greater amount to restore the engine intake manifold throttle inlet pressure. The amount of opening relative to the nominal amount of opening is the compressor recirculation valve offset. The compressor recirculation valve may be controlled according to a transfer function that includes the revised offset to improve compressor recirculation valve control.
The present description may provide several advantages. In particular, the approach may improve engine airflow control at low driver demand levels. Further, the approach may improve engine air-fuel ratio control during accelerator pedal tip-out conditions. Further, the approach may be applied to turbocharged or supercharged engines.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.